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Encyclopedia > Cardiac pacemaker

The contractions of the heart are controlled by electrical impulses, these fire at a rate which controls the beat of the heart. The heart and lungs, from an older edition of Grays Anatomy. ...


The cells that create these rhythmical impulses are called pacemaker cells, and they directly control the heart rate. Artificial devices also called pacemakers can be used after damage to the body's intrinsic conduction system to produce these impulses synthetically. Cells in culture, stained for keratin (red) and DNA (green). ... // Rhythm (Greek ρυθμός = tempo) is the variation of the duration of sounds or other events over time. ... Heart rate is a term used to describe the frequency of the cardiac cycle. ... This article is about a medical device which electrically stimulates the heart. ...

Contents


Control via the SA node

Schematic representation of the sinoatrial node and the atrioventricular bundle of His. The location of the SA node is shown in blue. The bundle, represented in red, originates near the orifice of the coronary sinus, undergoes slight enlargement to form the AV node. The AV node tapers down into the bundle of HIS, which passes into the ventricular septum and divides into two bundle branches, the left and right bundles. The ultimate distribution cannot be completely shown in this diagram.

Although all of the heart's cells possess the ability to generate these electrical impulses (or action potentials), a specialised portion of the heart, called the sinoatrial node, is responsible for the whole heart's beat. Bundle of His, from Grays Anatomy 1918 This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... Bundle of His, from Grays Anatomy 1918 This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... A. A schematic view of an idealized action potential illustrates its various phases as the action potential passes a point on a cell membrane. ... The sinoatrial node (abbreviated SA node, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart. ...


The sinoatrial node (SA node) is a group of cells positioned on the wall of the right atrium, near the entrance of the superior vena cava. These cells are modified cardiac myocytes. They possess some contractile filaments, though they do not contract. The sinoatrial node (abbreviated SA node, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart. ... Superior vena cava - Wikipedia, the free encyclopedia /**/ @import /skins-1. ... Myocyte is the technical term for a muscle cell. ...


Cells in the SA node will spontaneously depolarize, resulting in contraction, approximately 100 times per minute. This native rate is constantly modified by the activity of sympathetic and parasympathetic nerve fibers, so that the average resting cardiac rate in adult humans is about 70 beats per minute. Because the sinoatrial node is responsible for the rest of the heart's electrical activity, it is sometimes called the primary pacemaker. In biology, depolarization is the event a cell undergoes when its membrane potential grows more positive with respect to the extracellular solution. ... Grays FIG. 838– The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic plexuses. ... It has been suggested that Parasympatholytic be merged into this article or section. ...


If the S.A node doesn't function, or the impulse generated in the SA node is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become the heart's pacemaker, this is know as an ectopic pacemaker. These cells form the atrioventricular node (AV node), which is an area between the atria and ventricles, within the atrial septum. The atrioventricular node (abbreviated AV node) is the tissue between the atria and the ventricles of the heart, which conducts the normal electrical impulse from the atria to the ventricles. ...


The cells of the AV node normally discharge at about 40-60 beats per minute, and are called the secondary pacemaker.


Further down the electrical conducting system of the heart, the Bundle of His, the left and right branches of this bundle, and the Purkinje fibres, will also produce a spontaneous action potential if they aren't inhibited by other electrical activity. These tertiary pacemakers fire at a rate between 30-40 per minute. Purkinje fibers are located in the inner ventricular walls of the heart, just beneath the endocardium. ...


Even individual cardiac muscle cells will contract rhythmically on their own.


The reason the SA node controls the whole heart is that its action potentials are released most often; this triggers other cells to generate their own action potentials. In the muscle cells, this will produce contraction. The action potential generated by the SA node, passes down the cardiac conduction system, and arrives before the other cells have had a chance to generate their own spontaneous action potential. This is the normal conduction of electrical activity within the heart. The normal electrical conduction in the heart allows the impulse that is generated by the SA node of the heart to be propagated to (and stimulate) the myocardium (muscle of the heart). ...


Generation of action potentials

There are three main stages in the generation of an action potential in a pacemaker cell. Since the stages are analogous to contraction of cardiac muscle cells, they have the same naming system. This can lead to some confusion. There is no phase one or two, just phases zero, three and four.


Phase 4 - Pacemaker potential

The key to the rhythmical firing of pacemaker cells is that, unlike muscle and neurons, these cells will slowly depolarise by themselves. A top-down view of skeletal muscle Muscle is contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. ... Drawing by Santiago Ramón y Cajal of cells in the pigeon cerebellum. ...


As in all other cells, the resting potential of a pacemaker cell (-60mV to -70mV) is caused by a continuous outflow or "leak" of potassium ions through ion channel proteins in the membrane that surrounds the cells. The difference is that this potassium permeability decreases as time goes on, partly causing the slow depolarization. As well as this, there is a slow inward flow of sodium, called the 'funny' current. This all serves to make the cell more positive. In biological cells that are electrically at rest, the cytosol possesses a uniform electric potential or voltage compared to the extracellular solution. ... General Name, Symbol, Number potassium, K, 19 Chemical series alkali metals Group, Period, Block 1, 4, s Appearance silvery white Atomic mass 39. ... Another, unrelated ion channeling process is part of ion implantation. ... Integral membrane protein of the transmembrane type An Integral Membrane Protein (IMP) is a protein molecule (or assembly of proteins) that in most cases spans the biological membrane with which it is associated (especially the plasma membrane) or which, in any case, is sufficiently embedded in the membrane to remain... Drawing of a cell membrane A component of every biological cell, the selectively permeable cell membrane (or plasma membrane or plasmalemma) is a thin and structured bilayer of phospholipid and protein molecules that envelopes the cell. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ...


This relatively slow depolarization continues until the threshold potential is reached. Threshold is between -40mV and -50mV. When threshold is reached, the cells enter phase 0.


Phase 0 - Upstroke

Though much faster than the depolarisation caused by the funny current and decrease in potassium permeability above, the upstroke in a pacemaker cell is relatively slow compared to that in an axon. An axon, or nerve fiber, is a long slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neurons cell body or soma. ...


The SA and AV node do not have fast sodium channels like neurons, and the depolarisation is mainly caused by a slow influx of calcium ions. (The funny current also increases). The calcium is let into the cell by voltage-sensitive calcium channels, that opened when the threshold was reached.


Phase 3 - Repolarization

The calcium channels are rapidly inactivated, soon after they opened. Sodium permeability is also decreased.


Potassium permeability is increased, and the efflux of potassium (loss of positive ions) slowly repolarises the cell.


Control of heart rate

The heart gets its parasympathetic innervation from the vagus nerve. Signals from this nerve cause the heart rate to decrease. It has been suggested that Parasympatholytic be merged into this article or section. ... The vagus nerve (or pneumogastric nerve) is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem (within the medulla oblongata) and extends, through the jugular foramen, down below the head, to the abdomen. ... Nerves (yellow) Nerves redirects here. ...


Sympathetic stimulation comes from the cardiac nerves from the sympathetic chain. Activity in these nerves acts to increase heart rate. Grays FIG. 838– The right sympathetic chain and its connections with the thoracic, abdominal, and pelvic plexuses. ... The Sympathetic Chain is a series of interconnected sympathetic ganglia, belonging to the autonomic nervous system. ...


Sympathetic stimulation

When the SA node receives sympathetic stimulation, noradrenaline (norepinephrine) released from the nerve endings binds to β1-adrenergic receptors on the pacemaker cell membrane. Norepinephrine (INN) or noradrenaline (BAN) is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ... Norepinephrine (INN) or noradrenaline (BAN) is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ... The adrenergic receptors (or adrenoceptors) are a class of G-protein coupled receptors that is the target of catecholamines. ...


This binding causes cyclic AMP production within the cell. This directly increases the funny current, meaning sodium is continually entering the cell more quickly. Cyclic AMP also activates a protein kinase, that phosphorylates the calcium channels, increasing calcium conductance into the cell. Structure of cAMP Cyclic adenosine monophosphate (cAMP, cyclic AMP or 3-5-cyclic adenosine monophosphate) is a molecule that is important in many biological processes; it is derived from adenosine triphosphate (ATP). ... A protein kinase is an enzyme that modifies other proteins by chemically adding phosphate groups to them (phosphorylation). ...


Because both sodium, and calcium can enter the cell more quickly, the continuously natural depolarisation (phase 4) reaches threshold more quickly. So action potentials are generated more frequently.


It takes a while for the heart rate to increase after noradrenaline is released.


Parasympathetic stimulation

Acetylcholine (ACh) is released from the vagus nerve endings, and binds to muscarinic receptors on the pacemaker cells. The skeletal structure of acetylcholine The chemical compound acetylcholine, often abbreviated as ACh, was the first neurotransmitter to be identified. ... The vagus nerve (or pneumogastric nerve) is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem (within the medulla oblongata) and extends, through the jugular foramen, down below the head, to the abdomen. ... Muscarinic receptors are those membrane-bound acetylcholine receptors that are more sensitive to muscarine than to nicotine. ...


In the pacemaker cells, there are ACh sensitive potassium channels. These open in response to ACh binding, potassium ions leak out, and the cell gets hyperpolarised (more negative). The funny current is also reduced by ACh. This means sodium ions enter more slowly, and it takes longer for the cell to reach threshold. In addition, calcium influx is reduced; therefore, when threshold is reached depolarization takes longer. Thus the heart rate slows.


Unlike the sympathetic mechanism, the heart will slow quite soon after vagal stimulation. The vagus nerve (or pneumogastric nerve) is the tenth of twelve paired cranial nerves, and is the only nerve that starts in the brainstem (within the medulla oblongata) and extends, through the jugular foramen, down below the head, to the abdomen. ...


Hormonal effects

Noradrenaline and adrenaline (also called Norepinephrine and Epinephrine respectively) are both released into the bloodstream by the adrenal medulla. Norepinephrine (INN) or noradrenaline (BAN) is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ... It has been suggested that this article or section be merged with adrenalin. ... Norepinephrine (INN) or noradrenaline (BAN) is a catecholamine and a phenethylamine with chemical formula C8H11NO3. ... It has been suggested that this article or section be merged with adrenalin. ... In mammals, the adrenal glands are the triangle-shaped endocrine glands that sit atop the kidneys. ...


They have the same action on heart rate as direct sympathetic stimulation.


External references

  • Fox, S.I. (1993). Human Physiology, 4th ed.. Dubuque, IA, USA: Wm. C. Brown Publishers.

See also

Cardiovascular system - Heart - edit
atria (interatrial septum) | ventricles (interventricular septum) | valves (chordae tendinaepapillary muscle)

right heart(vena cavae, coronary sinus) → right atrium (fossa ovalis) → tricuspid valve → right ventricle → pulmonic valve 
(pulmonary artery and pulmonary circulation) The cardiac action potential is the electrical activity of the individual cells of the electrical conduction system of the heart. ... The normal electrical conduction in the heart allows the impulse that is generated by the SA node of the heart to be propagated to (and stimulate) the myocardium (muscle of the heart). ... The circulatory system or cardiovascular system is the organ system which circulates blood around the body of most animals. ... The heart and lungs, from an older edition of Grays Anatomy. ... In anatomy, the atrium (plural: atria) is the blood collection chamber of a heart. ... The interatrial septum is the wall of tissue that separates the right and left atria of the heart. ... In the heart, a ventricle is a heart chamber which collects blood from an atrium (another heart chamber that is smaller than ventricle) and pumps it out of the heart. ... Grays Fig. ... Grays Fig. ... Structure of the Chordae Tendineae Valves like the Tricuspid valve and the Semilunar valves in the heart are attached to the walls of the heart by cord-like tendons called chordae tendineae. ... In anatomy, the papillary muscles of the heart serve to limit the movements of the mitral and tricuspid valves and prevent them from being everted. ... Right heart is a term used to refer collectively to the right atrium and right ventricle of the heart. ... The superior and inferior venae cavae are the veins that return the blood from the body into the heart. ... An aortic sinus is one of the anatomic dilations of the ascending aorta which occurs at the aortic root, i. ... This page is about the muscular organ, the Heart. ... Found in the right atrium of the heart, the fossa ovalis is an embryonic remnant of the foramen ovale, which normally closes shortly after birth. ... In anatomy, the heart valves are valves in the heart that prevent blood from flowing the wrong way. ... The right ventricle is one of four chambers (two atria and two ventricles) in the human heart. ... In anatomy, the heart valves are valves in the heart that prevent blood from flowing the wrong way. ... The pulmonary arteries carry blood from the heart to the lungs. ... Pulmonary circulation is the portion of the cardiovascular system which carries oxygen-depleted blood away from the heart, to the lungs, and returns oxygenated blood back to the heart. ...


left heart: (pulmonary veins)left atriummitral valveleft ventricleaortic valve
(aortaaortic sinus and systemic circulation) Left heart is a term used to refer collectively to the left atrium and left ventricle of the heart. ... The pulmonary veins carry blood from the lungs to the left atrium of the heart. ... This page is about the muscular organ, the Heart. ... The mitral valve, also known as the bicuspid valve, is a valve in the heart that lies between the left atrium (LA) and the left ventricle (LV). ... In the heart, a ventricle is a chamber which collects blood from an atrium (another heart chamber) and pumps it out of the heart. ... The aortic valve is one of the valves of the heart. ... The largest artery in the human body, the aorta originates from the left ventricle of the heart and brings oxygenated blood to all parts of the body in the systemic circulation. ... An aortic sinus is one of the anatomic dilations of the ascending aorta which occurs at the aortic root, i. ... Systemic circulation is the portion of the cardiovascular system which carries oxygenated blood away from the heart, to the body, and returns oxygen-depleted blood back to the heart. ...


pericardium | epicardium | endocardium | myocardium The pericardium is a double-walled sac that contains the heart and the roots of the great vessels. ... Epicardium describes the outer layer of heart tissue (from Greek; epi- outer, cardium heart). ... In the heart, the endocardium is the innermost layer of cells, embryologically and biologically similar to the endothelium that lines blood vessels. ... Myocardium is the muscular tissue of the heart. ...


conduction systemcardiac pacemaker | Purkinje fibers | bundle of His | SA node | AV node The normal electrical conduction in the heart allows the impulse that is generated by the SA node of the heart to be propagated to (and stimulate) the myocardium (muscle of the heart). ... Purkinje fibers (or Purkyne tissue) are located in the inner ventricular walls of the heart, just beneath the endocardium. ... Heart cut away showing Bundle of His Schematic representation of the atrioventricular bundle of His. ... The sinoatrial node (abbreviated SA node, also called the sinus node) is the impulse generating (pacemaker) tissue located in the right atrium of the heart. ... The atrioventricular node (abbreviated AV node) is the tissue between the atria and the ventricles of the heart, which conducts the normal electrical impulse from the atria to the ventricles. ...


  Results from FactBites:
 
FDA Heart Health Online - Cardiac Pacemaker (implanted) (524 words)
A pacemaker is a small, battery-powered device that is implanted permanently into the body.
The pacemaker is connected to the heart through one to three insulated wires (leads) that are attached directly to the heart's chambers.
A pacemaker should not be implanted in people who cannot tolerate the device or the surgical procedure, or who are sensitive (allergic) to the exposed parts of the pacing system.
Pacemaker for bradycardia (690 words)
A pacemaker is a battery-powered device about the size of a pocket watch that sends weak electrical impulses to “set a pace” so that the heart is able to maintain a regular heartbeat.
This type of pacemaker varies its rate to cause the heart to beat faster when you are exercising to meet your body's increased needs or slower when you are at rest.
Temporary pacemakers are worn outside the body and attached to the heart by a wire threaded through a neck vein or leg vein or through the chest wall.
  More results at FactBites »

 
 

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